1. Field of the Invention
The present invention relates to a urethane composition for preparing a sheet transport roll, and to a sheet transport roll produced by employing such a urethane composition.
2. Description of the Art
Sheet transport rolls such as sheet feed rolls and transportation rolls for use in copying machines are generally required to have a high friction coefficient, a high abrasion resistance, and a high compression resistance. From the viewpoint of the abrasion resistance, a urethane roll composed of a polyurethane material is conventionally employed as the sheet transport roll. The urethane roll is generally excellent in abrasion resistance, but has a lower friction coefficient than a rubber roll. Therefore, the hardness of the urethane roll is reduced by adding a plasticizer to the polyurethane material or by reducing the NCO index of the polyurethane material to so as to increase the friction coefficient (Japanese Unexamined Patent Publication No. 63-77919 (1988)).
However, the urethane roll disclosed in Japanese Unexamined Patent Publication No. 63-77919 has an unsatisfactory friction coefficient. That is, it is impossible to provide a sufficiently high friction coefficient which is virtually equivalent to that of the rubber roll. If the hardness of the urethane roll is reduced to lower than 40, the urethane roll has an increased adhesion, so that paper dust is liable to adhere on the surface of the roll. This disadvantageously reduces the friction coefficient. If the NCO index is reduced, the crosslinking density is reduced, thereby disadvantageously deteriorating the compression resistance and the abrasion resistance.
In view of the foregoing, it is an object of the present invention to provide a urethane composition for a sheet transport roll which allows the sheet transport roll to have a high friction coefficient and an excellent abrasion resistance while maintaining the friction coefficient at a high level. It is another object of the invention to provide a sheet transport roll produced by employing such a urethane composition.
In accordance with a first aspect of the present invention to achieve the aforesaid object, there is provided a urethane composition for a sheet transport roll, the composition comprising: (A) a polyetherpolyol blend containing polytetramethyleneether glycol (PTMG) and polypropylene glycol (PPG) in a weight ratio of PTMG/PPG=99/1 to 50/50; (B) a polyisocyanate; and (C) a chain lengthening agent; the urethane composition in a cured state having a hardness of not smaller than 40, and a crosslinking density of 0.15 to 0.8 mmol/cm3 or an allophanate bond concentration of 0.03 to 0.07 mmol/g.
In accordance with a second aspect of the present invention, there is provided a sheet transport roll produced by employing the urethane composition according to the first aspect.
The inventors of the present invention have conducted intensive studies on polyol materials to provide a urethane composition for a sheet transport roll which has a high friction coefficient and an excellent abrasion resistance. Particularly, the studies have been directed to ether-type polyols, revealing that the specific polyether polyol blend (component (A)) containing PTMG (which has a high strength among other ether-type polyols) and PPG in a predetermined weight ratio provides favorable results. It has been noted that ester-type polyols fail to provide favorable results because the ester-type polyols are generally poor in hydrolysis resistance with ester bonds present in their molecular structures. The inventors have further conducted studies on the properties of the above urethane composition comprising the polyether polyol blend (component (A)), the polyisocyanate (component (B)) and the chain lengthening agent (component (C)) and, as a result, have found that the aforesaid object can be achieved by controlling the hardness and the crosslinking density or the allophanate bond concentration of the urethane composition in the cured state in the aforesaid predetermined ranges. Thus, the present invention has been attained.
The term xe2x80x9csheet transport rollxe2x80x9d herein includes a sheet feed roll (such as a pick-up roll, a feed roll or a retard roll) or a sheet transport roll (such as a transport roll) in a narrower sense, and further is intended to include a sheet transport belt and the like.
When the urethane composition includes an ion conductive agent in addition to the polyether polyol blend (component (A)), the polyisocyanate (component (B)) and the chain lengthening agent (component (C)), the ion conductive agent serves as an anti-static agent, so that paper dust can more effectively be prevented from adhering on a roll surface and the friction coefficient of the roll can more effectively be maintained at a high level.
When the urethane composition comprises a plasticizer and/or a hollow filler in addition to the components (A) to (C), the friction efficient advantageously can be increased.